The integration of diamond and GaN has been highly pursued for thermal management purposes as well as combining their exceptional complementary properties for power electronics applications and novel semiconductor heterostructures. However, the growth of diamond-on-GaN is challenging due to the high lattice and thermal expansion mismatches. The weak adhesion of diamond to GaN and high residual stresses after the deposition often result in the diamond film delamination or development of cracks, which hinder the subsequent device fabrication. Here, we present a new seed dibbling method for seeding and growing high-quality diamond films on foreign substrates, in particular on cost-effective GaN-on-Si, with significantly improved adhesion. Diamond films grown conformally on patterned GaN-on-Si presented high quality with significantly larger grains and a 95% sp(3)/sp(2) ratio, excellent interface between diamond and GaN, and lower residual stresses (as low as 0.2 GPa) compared to conventional methods. In addition, the method provided excellent adhesion, enabling a reliable polishing of the as-grown diamond films on GaN on Si without any delamination, resulting in smooth diamond-on-GaN substrates with subnanometer root-mean-square roughness. Diamond layers deposited via seed dibbling resulted in a 2-fold improvement in the effective thermal conductivity for GaN-on-Si with only a 20 mu m thick diamond layer. This method opens many new possibilities for the development of high-performance power electronic devices and integrated devices with excellent thermal management based on a diamond-on-GaN platform. In addition, this technique could be extended to other substrates to combine the outstanding properties of diamond with other kinds of devices.